Toluene–nitrite inhibition synergy of anaerobic ammonium oxidizing (anammox) activity

•Anammox reaction was inhibited by toluene; the level depended on the concentration.•Anammox sludge pre-exposed to nitrite alone or with toluene was more affected than toluene alone.•The toluene by itself did not severely inhibited the anammox process but in presence of NO2−.•The inhibition most likely was attributed to the nitrite–toluene inhibition synergy.

Anammox process is emerging as a promising biotechnology to remove ammonium (NH4+) from wastewater. Co-existing organic matters such as toluene could pose potential inhibitions on this process. In this study, the mechanisms of toluene–nitrite (NO2−) inhibition synergy of anammox activity (AA) was investigated in batch culture. In results, the AA (mmol N2-N Lliquid−1 h−1) in reference assays achieved values of 0.21 ± 0.0078. The toluene (0.1–3.0 mM) demonstrated different inhibitory effects on the AA and caused 50% inhibition (IC50) at 0.79 mM. The pre-exposure to toluene and NO2− or NO2− alone in the absence of NH4+ demonstrated a much severer inhibition, compared to toluene alone. The IC50 of the AA due to pre-exposure to NO2− lacking NH4+ was of 6.25 mM NO2−. These results indicated that toluene by itself did not severely inhibit the anammox process but did so in the presence of NO2−.